1. Field of the Invention
The present invention relates to an optical low pass filter and an imaging device utilizing such optical low pass filter.
2. Description of the Prior Art
An optical color imaging device of a type utilizing a single-plate type solid-state imaging element comprises a color filter array disposed on a solid-state imaging element to provide color signals. If rays of light reflected from an object being imaged contain a high spatial frequency component corresponding to the pitch between the neighboring color filters of the color filter array, the color imaging device outputs false color signals. The solid-state imaging device has a plurality of picture elements disposed discontinuously and regularly in a predetermined pattern and, therefore, if the rays of light reflected from the object being imaged contain a high spatial frequency component corresponding to each neighboring picture elements, the imaging device outputs false signal due to a recurrence of light.
Because of the foregoing, the optical system of the color imaging device of the type utilizing the single-plate type solid-state imaging element is provided with an optical low pass filter having a capability of avoiding generation of both of the false color signals and the false signals. While the optical low pass filter is generally comprised of three or more quartz plates and a single infrared cut-off filter laminated together, such optical low pass filter is known to be difficult to mass-produce and is therefore too expensive.
To overcome the difficulty in mass-productivity discussed above, the optical low pass filter comprising a diffraction grating has been suggested and is known in the art. One example of the imaging device utilizing the optical low pass filter comprising the diffraction grating is illustrated in FIG. 7, reference to which will now be made for the discussion of the prior art imaging device.
The prior art imaging device shown in FIG. 7 comprises a phototaking lens 21, an optical low pass filter 22 in the form of a diffraction grating and a solid-state imaging element 24 arranged in this specified order from an object side where an object to be imaged is situated. The solid-state imaging element 24 is enclosed in a package 23, and the optical low pass filter 22 is fitted to the front of the package 23 while concurrently serving as a protective glass for the solid-state imaging element 24.
It has been found that, when the imaging device of the type utilizing the optical low pass filter 22 as shown n and discussed with reference to FIG. 7 is aimed at a spot of light such as generated from, for example, an automobile headlight or an illuminating lamp, it is often observed that a generally ring-shaped flare shows up around an image of the spot light, accompanied by a reduction in quality of the image. A major cause of the occurrence of the ring-shaped flare around the image of the spot light is a total reflection occurring at a grating surface 22a, which forms a principal surface of the optical low pass filter 22, and another principal surface 22b thereof opposite to the grating surface 22a.